Using ROME to convert a syndication feed from one type to another

ROME represents syndication feeds (RSS and Atom) as instances of the com.rometools.rome.synd.SyndFeed interface. The SyndFeed interfaces and its properties follow the Java Bean patterns. The default implementations provided with ROME are all lightweight classes.

ROME includes parsers to process syndication feeds into SyndFeed instances. The SyndFeedInput class handles the parsers using the correct one based on the syndication feed being processed. The developer does not need to worry about selecting the right parser for a syndication feed, the SyndFeedInput will take care of it by peeking at the syndication feed structure. All it takes to read a syndication feed using ROME are the following 2 lines of code:

SyndFeedInput input = new SyndFeedInput();
SyndFeed feed = input.build(new XmlReader(feedUrl));

The first line creates a SyndFeedInput instance that will work with any syndication feed type (RSS and Atom versions). The second line instructs the SyndFeedInput to read the syndication feed from the InputStream of a URL pointing to the feed. The XmlReader is a character based Reader that resolves the encoding following the HTTP MIME types and XML rules for it. The SyndFeedInput.build() method returns a SyndFeed instance that can be easily processed.

ROME also includes generators to create syndication feeds out of SyndFeed instances. The SyndFeedOutput class does this generation. The SyndFeedOutput will generate a syndication feed of the feed type indicated by the SyndFeed object being output. The following two lines of code show how to create a syndication feed output from a SyndFeed instance:

SyndFeedOutput output = new SyndFeedOutput();
output.output(feed,new PrintWriter(System.out));

The first line creates a SyndFeedOutput instance that will produce syndication feeds. It can output feeds of any type (rss_0.9, rss_0.91, rss_0.92, rss_0.93, rss_0.94, rss_1.0, rss_2.0 & atom_0.3), the SyndFeed feedType property indicates the type. The second line writes the SyndFeed as a syndication feed into the application's output.

Following is the full code for a Java application that reads a syndication feed and converts it to other syndication feed type, writing the converted feed to the application's output.

package com.rometools.rome.samples;

import java.net.URL;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import com.rometools.rome.feed.synd.SyndFeed;
import com.rometools.rome.io.SyndFeedInput;
import com.rometools.rome.io.SyndFeedOutput;
import com.rometools.rome.io.XmlReader;

/**
 * It Converts any RSS/Atom feed type to a an RSS/Atom feed of the
 * specified type.
 * <p>
 * @author Alejandro Abdelnur
 *
 */
public class FeedConverter {

    public static void main(String[] args) {
        boolean ok = false;
        if (args.length==2) {
            try {
                String outputType = args[0];

                URL feedUrl = new URL(args[1]);

                SyndFeedInput input = new SyndFeedInput();
                SyndFeed feed = input.build(new XmlReader(feedUrl));
                feed.setFeedType(outputType);
                SyndFeedOutput output = new SyndFeedOutput();
                output.output(feed,new PrintWriter(System.out));

                ok = true;
            }
            catch (Exception ex) {
                System.out.println("ERROR: "+ex.getMessage());
            }
        }

        if (!ok) {
            System.out.println();
            System.out.println("FeedConverter converts between syndication feeds types.");
            System.out.println("The first parameter must be the feed type to convert to.");
            System.out.println(" [valid values are: rss_0.9, rss_0.91, rss_0.92, rss_0.93, ]");
            System.out.println(" [                  rss_0.94, rss_1.0, rss_2.0 & atom_0.3  ]");
            System.out.println("The second parameter must be the URL of the feed to convert.");
            System.out.println();
        }
    }

}